Contemporaneous Formation of Adjacent Porphyry and Epithermal Cu-Au Deposits Over 300 Ka in Northern Luzon, Philippines

Contemporaneous formation of adjacent porphyry and epithermal Cu-Au deposits over 300 ka in northern Luzon, Philippines Antonio Arribas, Jr. Geological Survey of Japan, Higashi 1-1-3, Tsukuba 305, Japan Jeffrey W. Hedenquist Tetsumaru Itaya Okayama University of Science, Okayama 700, Japan Toshinori Okada Rogelio A. Concepcio´n Lepanto Consolidated Mining Co., Makati, Manila, Philippines Jose S. Garcia, Jr.

ABSTRACT There is commonly a close spatial relation between porphyry Cu (6 Au) and high-sulfidation epithermal Cu-Au deposits throughout the world, although a genetic association has not been proven. Nowhere is this spatial association better seen than in northern Luzon, Philippines, where the Lepanto epithermal Cu-Au deposit overlies the Far Southeast (FSE) porphyry Cu-Au deposit, both world-class orebodies. Fresh rock and hydrothermal mineral separates yield K/Ar ages indicating that premineralization and postmineralization volcanism occurred at 2.2–1.8 Ma and 1.2– 0.9 Ma, respectively, and that the hydrothermal system was active from ;1.5 to ;1.2 Ma. K/Ar ages of alunite from Lepanto have the same range as those of hydrothermal biotite and illite from the FSE deposit, indicating that both epithermal and porphyry mineralization formed from an evolving magmatic-hydrothermal system that was active for about 300 ka. This temporal relation strengthens the argument for a genetic link between these two styles of ore deposit, and has implications for exploration. Where one style of mineralization is found, there is potential for the other nearby.

MANKAYAN MINERAL DISTRICT The Mankayan mineral district in the Central Cordillera of northern Luzon is one of the richest mining districts in the Philippine archipelago in terms of economic value and abundance and diversity of hydrothermal ore deposits. Within an area of ,25 km 2 (Fig. 1), the district contains several porphyry Cu-Au (Guinaoang, Palidan, and FSE) and epithermal precious- and base-metal deposits, both high-sulfidation (Lepanto) and low-sulfidation (Nayac, Suyoc) types. The Lepanto and FSE deposits are in the northern part of the district and show a close spatial relation; ore-grade porphyry Cu-Au mineralization in the FSE deposit occurs at ,800 m elevation, below and to the southeast of the Lepanto enargite-Au orebody, which extends for 3 km to the northwest from an elevation of 700 to 1200 m (Figs. 1 and 2). Combined, the two deposits have a metal content of .550 t (tonnes) Au and .3.6 Mt Cu (based on a cutoff grade of Cu and Au equivalent in value to 1.0 wt% Cu; Concepcio ´n and Cinco, 1989). The geology of the Mankayan district can be divided into four main lithologic groups (Fig. 1): (1) a volcanic to epiclastic basement

INTRODUCTION Porphyry and high-sulfidation epithermal deposits form in distinct geologic and geochemical environments within magmatic-hydrothermal systems associated with intermediate to felsic magmas (Hedenquist and Lowenstern, 1994). In porphyry deposits, disseminated or stockwork Cu (6 Au and/or Mo) mineralization forms within, or close to, an intrusive stock at depths .2 km and temperatures .350 8C. High-sulfidation epithermal deposits are located mostly within extrusive rocks, as veins or structurally controlled orebodies, and form at temperatures of ;150 –300 8C and depths ,1 km. Both deposit types share an association with advanced argillic alteration, and a close spatial relation has been noted (Sillitoe, 1983, 1991) and used in the exploration for new ore deposits. However, although suggested (Sillitoe, 1989), a genetic connection has not been proven, in part because no chronological data exist to support unambiguously a direct temporal relation between porphyry and epithermal mineralization. If a genetic connection can be demonstrated, this will have implications for the interpretation of oreforming processes related to high-level intrusions. We report the results of K/Ar dating of mineral separates from fresh and altered rocks from an area with closely related porphyry and epithermal mineralization in the Mankayan mineral district of northern Luzon, Philippines (Fig. 1). Coupled with the available geologic data on the Lepanto enargite-Au deposit and the Far Southeast (FSE) porphyry Cu-Au deposit, the K/Ar ages are used to determine the timing of magmatism and associated Cu-Au mineralization and the life span of the ore-forming magmatic-hydrothermal system.

Figure 1. Map of Mankayan district in northern Luzon (inset), Philippines, showing simplified geology and location and type of known hydrothermal deposits. Outlines of economically most important deposits (i.e., FSE [Far Southeast], Guinaoang, and Lepanto) are shown projected to surface (based on Garcia, 1991).

Data Repository item 9517 contains additional material related to this article. Geology; April 1995; v. 23; no. 4; p. 337–340; 4 figures.

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consisting of several units (i.e., Lepanto metavolcanic rock, Apaoan sedimentary rock, and Balili volcaniclastic rock) of Late Cretaceous to middle Miocene age; (2) a large Miocene intrusion (Bagon intrusion) of tonalitic to gabbroic composition; (3) a Pliocene dacitic pyroclastic and porphyry unit (Imbanguila hornblende dacite) that predates Cu-Au mineralization in the Lepanto and FSE deposits; and (4) an unaltered Pleistocene dacitic pyroclastic and porphyry unit (Bato hornblende-biotite dacite). The Imbanguila dacite forms a district-wide blanket of interlayered volcaniclastic and pyroclastic rocks and porphyritic lava flows. Near the FSE deposit, numerous dikes and two large (.300 m in diameter) volcanic vents filled with Imbanguila dacite porphyry and pyroclastic rock have been exposed by deep exploration drilling (Fig. 2). The Imbanguila and Bato dacites are lithologically and chemically (A. Arribas, unpublished data) similar and are likely to have been derived from a common parent magma. Lepanto and FSE Cu-Au Deposits The Lepanto enargite-Au deposit was worked for Cu prior to the sixteenth century from outcrops at ;1100 m elevation. The deposit is 3 km long and consists of a main zone of breccia and replacement mineralization along the Lepanto fault. Multiple veins branch off from the main zone at a sharp angle into both the hanging wall and the footwall (Garcia, 1991). The mushroom-shaped cross section of the enargite-Au orebodies is controlled by the intersection of the steeply dipping Lepanto fault and branch veins with the unconformity at the base of the Imbanguila dacite (Figs. 1 and 2). Lithologic variations in the host rocks also played an important role in the formation of the deposit, as shown by lenses of stratiform enargite-luzonite ore that resulted from replacement of detrital layers within volcaniclastic and sedimentary basement units (Garcia, 1991). The main characteristics of the Lepanto ores (e.g., Gonzalez, 1959; Garcia, 1991; Claveria and Hedenquist, 1994) are similar to those of other high-sulfidation deposits and include an association with massive and vuggy silica and hypogene advanced argillic alteration (alunite, pyrophyllite), an abundance of pyrite and high-sulfidation-state minerals, such as the Cu-sulfosalt minerals enargite and luzonite, and the presence of selenides, tellurides, and Sn-bearing minerals. Claveria and Hedenquist (1994) provided evidence that Au mineralization, accompanied by tennantite-tetrahedrite and chalcopyrite, followed enargite-luzonite deposition. Homogenization temperatures of fluid inclusions in enargite and quartz indicate that a dilute (,4 wt% NaCl equivalent) liquid cooled from ;280 to ;160 8C as it flowed northwest, away from the porphyry (Mancano and Campbell, 1994). The potential for high-temperature mineralization at depth and to the southeast of the Lepanto deposit was first suggested by Gonzalez (1959) on the basis on chemical and mineralogical gradients within the enargite-Au orebody. The FSE porphyry Cu-Au

deposit was discovered in 1980, and extensive drilling since then has resulted in the identification of an elongated, longitudinally bellshaped orebody that extends vertically downward for .1000 m from ;800 m elevation (i.e., from a depth of ;600 m below the present surface; Fig. 2). The distribution of metal grades is concentric around dikes and irregular intrusive bodies of melanocratic quartz diorite porphyry (Fig. 2). The Cu-Au ore (mainly bornite, chalcopyrite, and native Au) is rich in magnetite and occurs in extensively altered intrusive and country rocks. Hydrothermal alteration is characterized by a K-silicate (mainly biotite) core overprinted by illitechlorite alteration and late quartz-illite-sulfide and anhydrite veins. Outward, and in the upper parts of the deposit, alteration grades into argillic (kaolinite) and advanced argillic (pyrophyllite, diaspore, alunite) assemblages, subeconomic in grade, that mark the transition to the Lepanto deposit. Locally, enargite veinlets crosscut FSE porphyry-style alteration and mineralization. Fluid-inclusion studies indicate that high-temperature (.500 8C) saline brines were present during the evolution of the deposit (Concepcio ´n and Cinco, 1989). A breccia pipe that contains both porphyry and epithermal style mineralization cuts through the FSE deposit and overlying Imbanguila dacite (Concepcio ´n and Cinco, 1989). K/Ar AGES: SAMPLES AND RESULTS Samples for K/Ar dating (7 igneous and 24 hydrothermal mineral separates) were collected mainly from drill core and underground workings along a .4 km northwest-southeast cross section of the Lepanto and FSE deposits (Fig. 2). The Imbanguila and Bato units were dated using fresh igneous hornblende and biotite. Wellconstrained maximum ages for the epithermal and porphyry deposits were obtained by dating unaltered hornblende from drill-core segments of fresh Imbanguila dacite that graded, within several tens of metres, to intensely altered and mineralized dacite. Dating of the enargite-Au deposit was based on K/Ar ages of crystalline pink alunite, which occurs both as a product of hydrothermal alteration and as gangue in ore veins with kaolinite, quartz, pyrite, and Cu-bearing sulfides. The FSE porphyry system was dated using biotite from deep (,200 m elevation) remnants of early K-silicate alteration, and illite associated with late quartz-illite-sulfide veinlets. In addition, an illite-bearing sample was collected from the main zone of high-grade ($3.0% Cu equivalent) porphyry mineralization in the FSE core (east-400 level; Garcia, 1991). Details of laboratory procedures and error calculation for measurement of K and radiogenic 40Ar are reported by Itaya et al. (1991) and Okada and Itaya (1995), and in Table A.1 All ages are reported to the 95% confidence level (2s). 1 GSA Data Repository item 9517, Table A, Mineral K/Ar data for the Lepanto and FSE (Far Southeast) Cu-Au Deposits, Philippines, is available on request from Documents Secretary, GSA, P.O. Box 9140, Boulder, Colorado 80301.

Figure 2. Schematic cross section of Lepanto and Far Southeast (FSE) deposits along Lepanto fault (Garcia, 1991) showing major lithologic units, outline of enargite-Au and porphyry Cu-Au deposits, and most sample locations with K/Ar ages. Hornblende samples and one sample of biotite (1.18 Ma) are from fresh igneous rocks; alunite, illite, and other biotite samples are hydrothermal in origin.

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K/Ar ages of four hornblende separates from fresh Imbanguila dacite range between 2.19 6 0.62 and 1.82 6 0.36 Ma. These ages (Table A, Fig. 3) are older than those of hydrothermal minerals associated with the FSE and Lepanto deposits, i.e., biotite (1.45 6 0.04 to 1.34 6 0.05 Ma), illite (1.37 6 0.05 to 1.22 6 0.06 Ma), and alunite (1.56 6 0.29 to 1.17 6 0.16 Ma). On the basis of the K/Ar ages, the two deposits are contemporaneous and older than the unaltered Bato dacite (1.18 6 0.08 and 0.96 6 0.29 Ma). Dating of the quartz-diorite porphyries genetically related to FSE mineralization was not possible because all of these intrusions are hydrothermally altered. However, fresh hornblende from the matrix of volcanic breccia that crops out above Lepanto and contains lithic fragments with K-silicate alteration and porphyry-style mineralization (Concepcio ´n and Cinco, 1989) yielded an age of 1.43 6 0.21 Ma. The presence of the volcanic breccia and its K/Ar age suggest that there was some eruptive activity at the same time as the FSE mineralization and associated intrusive magmatism. TIMING OF MAGMATISM AND ORE FORMATION The analytical results are consistent with the field relations and indicate that hydrothermal Cu-Au mineralization took place in the Mankayan district in the middle of a Pliocene to Pleistocene event of dacitic-andesitic magmatism. Volcanism started in the late Pliocene with eruption of Imbanguila dacite—probably as early as 2.9 Ma in the northeast part of the district (Sillitoe and Angeles, 1985)—and continued until ;1.8 Ma. Between ;1.5 and 1.2 Ma, intrusion of quartz diorite led to extensive magmatic-hydrothermal activity and formation of proximal porphyry and distal epithermal Cu-Au mineralization—FSE and Lepanto, respectively. The location and morphology of the two deposits reflect differences in their formation: the FSE deposit has a concentric distribution around and within the intrusive body, in contrast to Lepanto, where ore is generally distal from the intrusion, located along the Lepanto fault and uncomformity at the base of Imbanguila dacite. Magmatism during formation of the deposits was mainly intrusive (see above) and it is likely that the comparatively narrow diorite dikes and irregular bodies found in the FSE deposit are

Figure 3. Radiometric ages for mineral separates from fresh and hydrothermally altered igneous rocks associated with Lepanto and Far Southeast (FSE) deposits. Samples are arranged arbitrarily by lithologic or mineral groups, and by age (older to left) within each group. Analytical uncertainty at 2s level is contained within size of symbols, except where indicated with bar. Illite separates were concentrated in